This invention relates to the technology of image exposure using two-dimensional spatial modulators such as a digital micro-mirror device. More particularly, the invention relates to an image exposing method and an image exposing apparatus that feature highly efficient utilization of light and which provide sufficiently uniform light to achieve uneven image exposure.
The digital image exposing systems currently used in various printers mostly depend on so-called laser beam scan (raster scan) for exposure, in which laser beams are deflected in a main scanning direction as a light-sensitive material and optics are moved relatively in an auxiliary scanning direction perpendicular to the main scanning direction, thereby performing two-dimensional exposure of the light-sensitive material with the laser beams modulated in accordance with the image to be recorded.
Various proposals have recently been made to perform digital image exposure using two-dimensional spatial modulators such as liquid-crystal displays (hereunder abbreviated as LCDs) and digital micro-mirror devices (hereunder abbreviated as DMDs) that are used as display means in displays and monitors. The basic principle of this exposing system is that the image displayed by means of a two-dimensional spatial modulator (hereunder referred to simply as a spatial modulator) is imaged on a light-sensitive material to expose it. For the purpose of rapid exposure, DMDs are more advantageous than LCDs since they permit faster modulation (response) and achieve more efficient utilization of light.
Digital exposing system using spatial modulators are disclosed in several prior patents, among which may be mentioned Unexamined Published Japanese Patent Application (kokai) No. 19662/2000. The exposing system disclosed in this patent creates a full-color image using light of three primary colors, R (red), G (green) and B (blue), and a spatial modulator (typically DMD). In this exposing system, the modulator (optics including it) and a light-sensitive material are scanned relatively in one direction of the pixel array of the DMD as the image displayed by the spatial modulator is scanned in synchronism in a corresponding direction, thereby achieving multiple exposures of the light-sensitive material. As a result, the gradation characteristics and the efficiency of light utilization are sufficiently improved to enable the outputting of images comparable in quality to photographs.
The use of digital technology is also under review in the printing platemaking art and computer-to-plate (CTP) systems have been proposed. In CTP, editing and related operations are performed with workstations and personal computers (PC), (digital) image data is generated for each of the created individual pages, and a PS plate is exposed with recording light modulated with the image data, thereby accomplishing direct platemaking from the input device such as PC to the PS plate.
In order to realize CTP by the existing technology, it is necessary to use a special PS plate that permits exposure with laser beams. However, compared to the conventional PS plates which are exposed by ultraviolet light (hereunder referred to simply as the conventional PS plate), the special PS plate is not only costly but also low in stability during development. Another difference concerns printability and depending on the type of prints, products of satisfactory quality are sometimes difficult to obtain. Furthermore, exposing of the special PS plate requires expensive light sources such as a visible light laser and a high-power infrared laser.
A CTP system has been proposed that allows for exposure of the conventional PS plate by using the aforementioned spatial modulator. In the system, an ultraviolet lamp is used as a light source and the image displayed by the spatial modulator is focused on the conventional PS plate to expose it. To be more specific, the DMD image is focused for a predetermined exposure time in a selected area of the conventional PS plate at rest, then the optics are moved to expose an adjacent area, and the same procedure is repeated. The time required to move the optics many times adds up to increase the time necessary for exposing the entire surface of the plate.
As a problem common to all exposing systems using spatial modulators that include not only the above-described CTP system but also the exposing system disclosed in Unexamined Published Japanese Patent Application No. 19662/2000, supra, the light incident on the light-sensitive material is low in the uniformity of its quantity and the resulting image has unevenness in density and other defects.
The present invention has been accomplished under these circumstances and has as a first object providing an image exposing method that can expose light-sensitive materials using a two-dimensional spatial modulator with high efficiency of light utilization and improved uniformity in light quality and which is particularly suitable for use with CTP systems that employ the conventional PS plate.
The present invention has also as a second object providing an image exposing apparatus that implements the image exposing method of the present invention in addition to the first object.
In order to attain the first object described above, the first aspect of the present invention provides an image exposing method, comprising the steps of: emitting light from a light source matching the spectral sensitivity of a light-sensitive material; modulating the thus emitted light with a two-dimensional spatial modulator; and exposing the light-sensitive material with the thus modulated light wherein the light-sensitive material and the two-dimensional spatial modulator are relatively scanned in a direction of one pixel column in the two-dimensional spatial modulator, as well as, in synchronism with this scanning, an image created by the two-dimensional spatial modulator is also scanned in the direction so as to perform multiple exposures of the light-sensitive material and wherein exposure with part of pixels in at least one of pixel columns is rested always irrespective of the image to be recorded such that a maximum quantity of exposure with each of the pixel columns in the direction is equal in each pixel column.
Preferably, successive measurements of quantity of light from a predetermined number of pixels are performed for all pixels and the pixels for which exposure is rested always are determined from results of the measurements.
Preferably, the two-dimensional spatial modulator is a digital micro-mirror device and quantities of light from the predetermined number of pixels or pixel columns are measured with reflected light in non-exposure mode.
Preferably, the two-dimensional spatial modulator is a digital micro-mirror device and quantities of light from the predetermined number of pixels or pixel columns are measured with reflected light in non-exposure mode.
In order to attain the second object described above, the second aspect of the present invention provides an image exposing apparatus, comprising: a light source for emitting light matching the spectral sensitivity of a light-sensitive material; a two-dimensional spatial modulation for modulating the light emitted from the light source in each of two-dimensional pixels; an exposing unit for exposing the light-sensitive material with the light modulated by the two-dimensional spatial modulator; wherein the exposing unit comprises: a first scanning unit for relatively scanning the light-sensitive material and the two-dimensional spatial modulator in a direction of one pixel column in the two-dimensional spatial-modulator; a second scanning unit for also scanning an image created on the light-sensitive material by the light modulated in each of the two-dimensional pixels by the two-dimensional. spatial modulator in the direction in synchronism with relative scanning so as to perform multiple exposures of the light-sensitive material; and control unit for controlling the two-dimensional spatial modulator in such a way that exposure with part of pixels in at least one of pixel columns is rested always irrespective of the image to be recorded such that a maximum quantity of exposure with each of the pixel columns of the two-dimensional pixels of the two-dimensional spatial modulator in the direction is equal in each pixel column.
Preferably, the first scanning unit is a scanning unit for moving the light-sensitive material with respect to the two-dimensional spatial modulator.
Preferably, the first scanning unit is a drum scanner.
Preferably, the second scanning unit is a switching unit for switching on or off exposure of the image created by the two-dimensional spatial modulator.
Preferably, the two-dimensional spatial modulator is a digital micro-mirror device.